Electrical load transfer switches, such as shown in U.S. Pat. No. 4,034,170, are conventionally employed to switch an electrical load between a normal power supply, such as the utility company lines, and an emergency power supply, such as a stand-by diesel engine powered generator. Such devices are typically incorporated into the electrical system of large buildings to operate elevators, emergency lighting and other equipment in the event of a failure of the electric utility company's power. The transfer switches either consist of separate interconnected single pole-single throw switches for the emergency and normal power sources or a single pole-double throw switch for each phase of the electric power being controlled.
Such transfer switches may either be manually operated or automatically controlled by an electric circuit that detects a fault in the normal electric power source. The control circuit senses an undervoltage condition or a complete lack of power from the utility company, for example, and energizes a solenoid or other mechanism activating the switch to connect the building's emergency circuits to a generator. The sensing circuitry may also automatically start the generator.
Because such devices typically must withstand relatively large electric currents, for example many thousand amperes under short circuit conditions, several electro-dynamic effects must be taken into account in the switch design. The first affect to be reckoned with is the constriction of the current path between the contacts of the switch. Large electric currents flow to and from the constriction in opposite directions, closely spaced to each other. This creates a force directly proportional to the square of the current flowing through the contacts which tends to move the switch contacts apart. In order to overcome this contact separation force, previous switch assemblies have incorporated large spring mechanisms to exert forces which counteract this force.
Another electro-dynamic force that has to be taken into account is the force from the magnetic fields that are established around the conductive elements of the switch. It is well known that currents flowing in opposite directions through two parallel conductors create electromagnetic fields having flux lines running in opposite directions. The opposed lines of flux tend to force the two parallel connectors apart. If a switch is designed with currents flowing in opposite directions through parallel contact arms, the electro-magnetic fields create a force which tends to separate the contacts. Heretofore, large spring assemblies have typically been incorporated into the switch mechanism to counteract these electro-magnetic forces.
These forces which tend to separate the contacts, commonly referred to as the "blow-off" force, have been used in circuit breakers to enhance the separation of circuit breaker contacts in the presence of excessive currents.
A corollary electro-magnetic force causes an attraction of two conductors when currents flow in the same direction through two parallel conductors. This attraction force has been used to hold the switch contacts together and is commonly referred to as a "blow-on" force. An example of a switch which takes advantage of the blow-on force is shown in U.S. Pat. No. 4,467,301. In that device, the fixed contacts are formed as part of a loop assembly so that a part of the final switch contact is parallel to the movable contact. The current flows in the same direction through the part of the fixed switch contact and the movable contact thereby creating a blow-on force. The blow-on force due to the electro-magnetic fields is also proportional to the square of the current flowing through the switch. Therefore, the blow-on force is proportional to the blow-off force. These forces counteract each other in a properly designed switch mechanism which takes advantage of the blow-on forces. The result is that the contacts remain closed under a high fault current.
However, previous loop type blow-on switch contacts such as in U.S. Pat. No. 4,467,301 did not lend themselves to a single pole-double throw type mechanism as used in transfer switches.